A multiple-access communication system is a system in which multiple users can simultaneously communicate over the same channel. A code division multiple access (CDMA) system is a multiuser system in which signals of different users are spread over a wide frequency band using different spreading codes. The despreader in such a system uses the spreading code of each individual user to despread that user's signal and obtain the originally transmitted data.
The widespread acceptance of wireless technologies has triggered a huge demand for bandwidth that is expected to grow well into the future [1]. The traditional approach to ensure coexistence of diverse wireless systems is spectrum licensing. However, after many years of spectrum assignment to meet the ever increasing demand, the Federal Communications Commission's (FCC's) frequency allocation chart [2] now shows a heavily crowded spectrum with most frequency bands already assigned to different primary users for specific services. Cognitive radios have been proposed as a technology to allow secondary wireless devices to coexist with the primary users of the spectrum without disrupting the communications between them.
The underlay approach allows concurrent primary and secondary transmissions [1,3]. The underlay system for cognitive radio network was proposed using ultra wide band (UWB) signaling in [4,5]. CDMA systems with high spreading factors may also be considered as underlay networks. Underlay systems protect primary users by requiring the secondary signals to operate at transmit powers that result in interference levels that are below the acceptable noise floor for the primary users of the spectrum [6].
The opportunistic approach in cognitive radio is where spectrum sensing occurs followed by spectral allocation for transmission by secondary users in sub-bands that are perceived to be empty. The detection process suffers from both a probability of not detecting an active primary user or mistakenly marking a sub-band as occupied by a primary user. Furthermore, the spectrum sensing must continue periodically to detect the event of primary users arriving at, or leaving, the network. This requires dynamic re-allocation of spectrum and secondary users will regularly change their transmit frequencies, and reduces throughput due to the quiet periods required for sensing. Oftentimes, only highly fragmented bands are available. In [7], adaptively changing bandwidth and power of sub-bands in multi-carrier CDMA is proposed to improve performance, but the spectral map is assumed to be perfectly known to the secondary network.
The underlay approach may be advantageous over the opportunistic approach due to the above and particularly in some specific scenarios. For example, it is preferable if spectral conditions are changing very fast or the primary user is highly mobile. Its main disadvantage is the strict transmit power and large bandwidth spread requirements on the secondary users.
In CDMA systems, including those with underlay networks and those with only a single class of users, it is desirable to assign spreading codes to users in a manner that minimizes multiple access interference. It is also required to maintain service parameters, such as bit error rate, at an acceptable level. Accordingly, there exists a need for interference-minimizing code assignment and system parameter selection in CDMA networks.